Reliable Design of Ammonia and Urea Plants201 Reliable Design of Ammonia and Urea Plants During the...

Technical Paper May 2015

Reliable Design of Ammonia and Urea Plants

Authors Pasquale Talarico CASALE GROUP – Switzerland Andrea Scotto CASALE GROUP – Switzerland Summary During the design of ammonia and urea plants many aspects have to be taken into account besides process performances. The plant reliability is a factor as important as process performances in determining the future plant profitability. To design a reliable plant it is necessary to take into account many aspects, such as the process mechanical design of all parts, instrumentation, process control, and the plant layout. It is also essential to have a good quality control in every phase of the project. This paper describes the design choices that are crucial in order to reach the highest standard of reliability in these plants.

Reliable Design of

Ammonia and Urea Plants

During the design of ammonia and urea plants many aspects have to be taken into account besides

process performances. The plant reliability is a factor as important as process performances in

determining the future plant profitability. To design a reliable plant it is necessary to take into

account many aspects, such as the process mechanical design of all parts, instrumentation, process

control, and the plant layout. It is also essential to have a good quality control in every phase of the

project. This paper describes the design choices that are crucial in order to reach the highest standard

of reliability in these plants.

Pasquale Talarico

CASALE GROUP – Switzerland

Andrea Scotto CASALE GROUP – Switzerland

Introduction

s Reliability is a broad scope,

understanding what it is, comes first.

Exploring the literature, it is possible to

find many definitions of reliability, like:

The ability of a system or component to

perform its required functions under stated

conditions for a specified period of time.

The idea that something is fit for a purpose

with respect to time;

The capacity of a device or system to

perform as designed;

The resistance to faults or failures of a

device or system;

The ability of a device or system to perform

a required function under stated conditions

for a specified period of time;

The probability that a functional unit will

perform its required function for a specified

interval under stated conditions.

The ability of something to "fail well" (to

fail without catastrophic consequences and

is restorable in a reasonable period of time).

In our opinion the more accurate statement is:

"The high reliability of plants is a choice and

not an accident of fortune".

In fact, once a plant is designed and built there

is very little that can be done to reduce

operating costs and reliability is a significant

part of them, because they are substantially

established during the plant's design. If low

operating costs is the target, Figure 1 makes it

clear that they are designed into the plant and

equipment during feasibility, design and

construction phases.

A

2012011 AMMONIA TECHNICAL MANUAL

Figure 1. Life Cycle Cost Commitments

A part from general statements and standard

activities which are usually performed during a

plant's design phase like HAZOP, SIL analysis

etc., this paper has the objective to provide

additional and unusual approach to plant's

reliability as it may be seen from the eyes of a

process licensor.

Therefore the paper describes the reliable design

approach for Ammonia and Urea plants that

should be undertaken by the various engineering

disciplines through the illustration of some

specific examples which are out of the well

know and recognized critical area of these type

of plants.

As an example, it is well known to the majority

of the specialists of the fertilizer sector that

corrosion phenomena are of paramount

importance for the reliability high pressure Urea

plants but in case a low-pressure carbonate

pumps fails the overall urea plant may stop as

well with the same result of losing the overall

production.

The Ammonia plant is a sequence of different

units. It is important for reliability that all of

them, including the sections or equipment

generally considered less critical like process

condensate pumps, turbine condensate pumps,

medium pressure low temperature exchangers,

compressor seal gas system, operate according

to recognize state of the art because their failure

will also result in an undesired plant shut-down.

Having this approach in mind, this paper will

provide some specific examples related to the

more significant disciplines which are typically

involved in the design of Ammonia and Urea

plants. In particular it will illustrate the process

design as well as the mechanical, machinery,

instrument and piping design.

1. Process Design

The process design has a fundamental role for

the operating reliability of the plants: as an

example the proper selection of operating

conditions, the control system and the material

of construction is a paramount step in the further

development of the project.

Furthermore, the operating flexibility originated

by the proper selection of a scheme that can

allow to safely operate the unit within a

sufficiently wide span of pressure, temperature

and fluid composition ranges is an aspect that

can boost the plants reliability. In fact this

feature, under certain conditions, can prevent

the plant to trip in case of human error or

instrumentation failure.

Entering more in the specific approach

highlighted above the following paragraphs

provide some specific examples of how the

process design can contribute to Ammonia and

Urea plant reliability.

Ammonia Process Design

The ammonia plant is a quite complicated

petrochemical gas plant that utilizes seven

different chemical reactions and fourteen unit

operations working on range of operative

condition that spans from cryogenic (-33°C /

-27°F) up to elevated temperature (1000°C /

1832°F ) as well as from low pressure (1 bar /

15 psi) to high pressure (150 bar / 2175 psig).

The know-how to manage all the wide range of

202 2011AMMONIA TECHNICAL MANUAL

operative condition and fluids to be handled is

of critical importance.

Such know-how covers all the engineering

disciplines who must do the following starting

from the process design:

Define the most appropriate flow scheme

and operative parameters of each

equipment/unit according to the "state of

the art" of the available proven technology.

Define the most appropriate design

condition and material selection for all

equipment/units of the whole plant.

Define the sparing philosophy for all

machines.

Define the required instrumentation for

monitoring and control the plant operation

within the defined operative parameters

along the plant.

Define the required instrumentation and

emergency shut-down system for

preventing operation outside the defined

operative parameters that may offset the

reliability and safety operation of any

equipment, units or plant sections.

Define the safety relief system able to

manage any possible overpressure that may

be caused by plant mal-operations, utility

failure or failure of process control and

emergency interlock system.

As an example the design of a suitable steam

network system and in particular the control

system for the let-down station and associated

valves is important for minimizing the network

steam pressure fluctuation in case of partial

unit/section shut-down avoiding to triggering

undesired total plant shut-down.

Urea Process Design As it is well know the modern urea processes

are the recycle type of process, therefore a

temporary or sudden inefficiency of the

synthesis loop, or more in detail of the urea

reactor, would have a significant impact on the

overall urea plant performances and under

certain conditions may even lead to a plant shut-

down.

Typically the best way to overcome this

problem is to provide a stable recycle flow of

carbamate to the high pressure section thus

preventing the further increase of water in the

synthesis loop that typically ends up in further

lowering the reactor efficiency.

This kind of problem can be safely and

environmentally addressed providing a

pressurized carbamate tank with a sufficient

hold up to manage the described transient

situation. In fact the excess of carbamate

generated by the synthesis loop can be stored

under pressure, thus preventing unnecessary

ammonia venting to atmosphere, maintaining or

even reducing the carbamate flow to the reactor.

Once the conditions of the reactor are recovered

the excess carbamate can be slowly recycle to

the synthesis without affecting the plants

operation and production.

2. Pressure Equipment Design

The pressure equipment in petrochemical

process plants is designed to last from 10 to

more than 25 years, according to the operating

conditions, without maintenance and it is

usually not spared. Therefore their reliability is

one of the key factors for the entire plant

reliability since their failure usually imply a

plant shut-down and quite often personnel and

plant safety is concerned.

Reliability concerns in the design and

construction of equipment have been

incorporated in the requirements of various

codes or practice of design, which are based

upon the wide experience and knowledge of

professional experts and specialists in the

industry, backed up by the experience of local

plant managers, engineers and operators who


have direct experience in the relevant plant

operation.

But equipment reliability has to be taken into

special consideration from the beginning of the

whole project and should be developed in each

step of the project as an integrated aspect of

design, not considered as a separate issue

referring only to the mechanical features.

This is particularly true for Ammonia and Urea

plants that involve high pressure, elevated

temperature and corrosive conditions, often

mixed together, which requires reliability

considerations for equipment to be already

incorporated in the process concept.

The most important factors affecting equipment

reliability are the metallurgical and mechanical

features and the aspects to be considered are:

Definition of the best design solutions for

the defined conditions and material

selection.

Full development of the defined design

solutions according to the best available

technologies.

Selection of the most appropriate supplier

of equipment, with particular care for the

critical items, which can assure not only the

best performance but also the most proven

design from a reliability and safety point of

view.

Perform all the required test at the

equipment supplier to guarantee a correct

manufacturing quality.

Proper installation and commissioning.

Compliance with the monitoring and

maintenance procedures to preserve

equipment characteristics and prevent

failures and early deterioration.

In all these steps reliability aspects shall be one

of the leading targets.

It should be noted that metallurgical and

mechanical aspects cannot be treated separately,

since any material choice involves specific

mechanical features and vice versa.

Whereas the fundamentals are common,

Ammonia and Urea equipment shall face

different conditions which impose significant

differences in their design to reach the desired

reliability.

Equipment for Ammonia Plants

The Ammonia process is characterized mainly

by gaseous fluids, mixtures of hydrogen,

methane, steam, air, carbon mono and dioxides

and ammonia. Gaseous mixtures are generally

not corrosive, therefore carbon and low alloy

steels are extensively used in ammonia plants,

but if they are at high temperature and/or high

pressure, they can become highly aggressive

and require special design and material selection

to assure reliable operations. The most

challenging sections are the reforming and the

synthesis sections.

In the reforming section the high temperatures,

in some part above 900°C (1650°F), are a

challenge in itself, but the presence of hydrogen,

carbon mono and dioxide and steam in the gas

can lead to phenomena such as metal dusting,

which can have catastrophic consequences.

The design of equipment in this section involves

very different solution and materials, from the

use of special metals for high temperature, to

the extensive use of refractory, to water

jacketing, from the selection of high alloys

resisting to metal dusting, to design features that

avoid the temperature range in which metal

dusting is possible.

The most critical items are the primary

reformer, the secondary reformer, the

connecting transfer line and the downstream

boiler system. For those items in addition to the

needed precautions during design, construction,

testing and installation, a very specific

experience along the supply chain from the


designer to the Manufactures is required to

achieve the expected reliability.

In the synthesis section the material selection is

mainly governed by two phenomena related to

the particular conditions of gas that contains

hydrogen and ammonia at high temperature and

pressure. These conditions require a specific

material selection and equipment design, to

avoid problems related to the two concurring

phenomena: hydrogen attack and nitriding.

Hydrogen Attack

The term hydrogen attack is used to indicate

several phenomena linked to the damages

caused by hydrogen. The most considerable

ones for ammonia plants are:

High Temperature Hydrogen Attack, which

in an Ammonia plant typically occurs in

areas such as secondary reformers, waste

heat boilers/exchangers, high temperature

shift converters, methanator and synthesis

loop equipment (Figure 2).

The conditions under which hydrogen

attack can occur are defined by a set of

empirical curves, namely the Nelson

Curves, reported on API 941, which set

permissible operating limits as a function of

steel type, hydrogen partial pressure and

temperature.

Figure 2. High temperature hydrogen attack

Hydrogen assisted cracking, which has also

been detected in Ammonia plants vessels,

in particular where the component had not

been adequately post weld stress relieved

during fabrication. For this reason

Hydrogen assisted cracking can occur at

service temperatures well below the

relevant Nelson curve. Generally it occurs

in low alloy with a chrome level above 2%,

typically utilized in the hottest part of the

synthesis section.

Hydrogen disbonding could affect

components where an overlay of stainless

steel or Inconel 600 has been applied on a

carbon or low alloy pressure resistant body,

usually to provide protection against high

temperature hydrogen attack, nitriding and

hot corrosion from sulphides (H2S) typical

of Ammonia plants.

There have been numerous instances where

equipment incorporating this design has

suffered from cracking at the interface

between the austenitic layer and the base

metal after being in service for a period of

time.

It could be connected to entrapped at the

overlay-base metal interface during a

cooling down cycle or to a phenomenon

called 'carbon migration' with carbon

migrating from the alloy steel and

concentrating at the austenite-alloy steel

interface to produce a high hardness, crack

susceptible martensitic zone.

This degradation mechanism is not

uncommon in Ammonia Plants with an

Inconel 600 overlay on low alloy with a

chrome level above 2%.

Nitriding

In the presence of a hot ammonia atmosphere,

above a certain temperature depending on the

type of steel, ammonia reacts with iron to form a

hard and brittle Fe-N inter-metallic layer. This

phenomenon is called Nitriding. Nitriding

develops on low alloy steels and on stainless


steels, however, on the latter at a much reduced

rate and higher temperatures compared with low

alloy steels.

On pressure equipment this layer does not cause

any problem until it remains compact and does

not crack, but since a brittle material will reach

more easily its rupture limit, in areas of stress

concentration cracks can occur, exposing further

surface to the nitriding atmosphere. This

propagation, cycle after cycle, can lead to the

component failure (Figure 3).

Figure 3. Section - Nitriding progress through

cracks

For this reason in ammonia atmosphere, usually

above 370-380°C, (700-715°F) carbon steel and

low alloy steel are not used in contact with fluid

and replaced with austenitic stainless steel or

even non-ferrous alloy.

Nitriding occurs also on stainless steel usually

above 400°C (750°F), but considering the high

ductility of stainless steel and their low nitriding

rate, their use guarantees an acceptable

reliability in this situation. Only the thinnest

components at the higher temperature usually

require the selection of material such as Inconel

600 which is fully resistant to nitriding.

Synthesis Sections

All the equipment is critical in the synthesis

section, but the most critical ones are the

ammonia converter and downstream

exchangers, because they see the combined

highest temperatures and the highest ammonia

content.

In particular reliability is essential for the

ammonia synthesis converter, which is the

reactor with the longest run between catalyst

changes, since it must run for 10-15 years.

Ammonia catalyst, once reduced, should not

come into contact with oxygen, since it is highly

pyrophoric. Therefore converters should operate

between catalyst changes without repair or

inspection.

In general, since the use of stainless steel or

higher alloy steels would be too expensive,

these items are made from low alloy steel,

selected according to the Nelson curves to resist

hydrogen attack.

To avoid nitriding usually either the temperature

of the pressure components is kept below 370°C

or a lining with a nitriding resistant material is

applied, this second solution being less reliable

due to the possible occurrence of hydrogen

disbonding.

The first solution usually calls for specific

internal design, direct connections between

equipment, which imply specific knowledge and

design solutions, but guarantees a higher

reliability. Since the detrimental effect of the

process environment on the materials increases

in direct proportion to the temperature, the latter

needs to be kept as low as possible on high-

pressure parts of the process. Reliability is

increased keeping materials at a lower

temperature by means of suitable thermal

insulation and/or gas flushing.

This concept inherently increases the safety of

high-pressure parts.

Even with the improved construction

technology, some recent cases involving an

additional hot wall ammonia converter suffered


from weld cracking on the girth weld between

shell and head, showing that the philosophy of

keeping cold the pressure components is still the

one guaranteeing the highest reliability.

Also the synthesis loop heat exchangers located

downstream the ammonia converter are subject

to the same concerns as the ammonia converter,

especially the first exchanger after the converter

outlet.

Whatever exchanger is selected either a steam

super-heater, or a boiler, or boiler feed water

pre-heater, or a gas-gas exchanger, the design

philosophy should be the same: keep

temperature low. Of course the design solutions

to achieve this target are specific to each type of

exchanger.

The possibility to directly connect the exchanger

to the converter, avoids the need for high-

pressure, high-temperature piping, which is a

usual source of trouble and leakage, and saves,

at the same time, material and pressure drop.

The arrangement of the exchanger itself (for

example, whether the gas is on the shell side or

the tube side) depends on the plant

configuration, but the use of U-tube type greatly

increase the exchanger reliability over the fixed

tube-sheet type. All design possibilities to

reduce stresses shall be considered, such as

using fountain-type U-tubes, in which the tubes

are arranged radially instead of parallel to each

other, as in a conventional U-tube exchanger,

permitting a symmetrical thermal stress

distribution, while no part of the pressure-

retaining wall of the gas channel is exposed to

the hottest gases.

For kettle type boilers (Figure 4), the horizontal

disposition usually is more reliable than the

vertical ones, where boiling water contaminants

deposit on the tubesheet causing corrosion.

Figure 4. Horizontal type synthesis boiler

directly connected to ammonia converter

When super-heaters are considered, since tube

material is subject to nitriding temperatures,

high alloy material specifically developed for

stress corrosion resistance should be preferred

over stainless steel, because of the high risk of

stress corrosion inherent in this material.

Other Sections

The same principles for design and materials

selection should be used to guarantee the

reliability of the equipment in all other sections

of the plant, although temperature and pressures

are lower.

However it should be considered that each plant

has its own specificities, different ambient

conditions, local rules, types of feed gas and

therefore the approach one design fit all is not

appropriate to ensure the best design and

reliability.

Special consideration should be given in these

aspects to the CO2 removal section, where the

possibility to choose different solution and the

different corrosiveness involved in each choice

is a key aspect of the plant reliability.


Equipment for Urea Plants

The urea process is characterized by the

presence of quite aggressive process fluids like

ammonium carbamate, ammonium carbonate

and urea solution, which require the use of

special stainless steels, especially under

synthesis conditions (high pressure and high

temperature). In principle, all urea processes

utilize stainless steel, but while the low-

pressure, evaporation, vacuum and wastewater

treatment sections use standard commercial

stainless steels (AISI-304L or AISI-316L), the

synthesis loop is manufactured with special

steels properly studied for urea application.

It has to be kept in mind that different process

choices greatly affect the material selection of

the equipment; therefore, the same equipment in

a different process scheme may require a

completely different approach.

The most common type of special material for

the Urea synthesis is 316L 'urea grade' (UG)

steel.

Figure 5. Active corrosion in 316L Urea grade

material

This material is widely used in urea plants,

mainly because of its excellent weldability, fair

corrosion-resistance and relatively low cost.

However, the large amount of passivation air

required and limited corrosion-resistance in

harsher conditions prevents its application for

the most critical components (Figure 5).

The super-austenitic stainless steel type 25Cr-

22Ni-2Mo is an upgrade of 316L-UG. It has

better corrosion-resistance, higher passivation

and re-passivation capacity and excellent

weldability.

This material is generally the basic selection

when the presence of passivation air allows

stainless steels to be used.

This type of material has been used for many

years in urea plants and has proven to be

reliable within its operating limits. The

performance of this material has always been

satisfactory, even in the most critical items such

as the HP stripper.

As for 316L-UG, since the introduction of 25-

22-2 CASALE has developed the relevant

specifications aimed to define the technical

requirements, acceptability criteria and

qualification tests for base material to be used in

CASALE equipment as well as all the

requirements for welding 25-22-2. These

specifications are the result of CASALE

experience in the field of urea and are

continuously updated on the basis of the

improved experience and the technical progress.

While austenitic stainless steels are susceptible

to stress corrosion cracking (SCC) by chlorides

which may be present in the utility fluid,

especially in carbamate condensers, duplex

stainless steels have high resistance to SCC.

They have been used in some plants for urea

synthesis components such as mixers, valves

and piping for more than 30 years.

But experience shows that common duplex

stainless steels are not suitable for the harsher

conditions, such as stripper or reactor

components.


Specific duplex steels have been developed for

the urea process conditions, to extend their use

to more critical components. These types of

duplex usually have high levels of chromium,

which is favorable to passivation of steel. As a

consequence, duplex stainless steel is easier to

passivate than austenitic steels, which means

duplex stainless steel requires less dissolved

oxygen compared to austenitic stainless steel.

However, it has to be remembered that the

dissolved oxygen content in the liquid phase

varies, even if the oxygen feed is constant. The

excessively low dissolved oxygen content, not

oxygen feed, could cause active corrosion even

in high-chromium duplex stainless steel.

Therefore the quantity of oxygen feed must be

determined on the basis of an analysis of the

complete system.

The passive corrosion rate of duplex stainless

steel developed for urea application is generally

comparable to that of 25Cr-22Ni-2Mo.

Special duplex materials developed to increase

their already high resistance to erosion are

selected for valves plugs, ejector nozzles,

mixers and similar application.

Titanium and Zirconium are reactive metals

which owe their corrosion-resistance to its

ability to passivate. The addition of oxygen to

the process is not required to passivate these

metals. Moreover, they are neither subject to

intergranular corrosion in urea synthesis

conditions nor to stress corrosion cracking.

Therefore they are ideal candidates for the most

critical components in the urea synthesis.

Titanium has been widely utilized in the past.

Equipment made of this material is suitable for

the highest temperature, even in absence of

oxygen, but it is expensive and its life is limited

due to erosion, especially in the case of

strippers. In addition, reliability is lower

compared with stainless steel equipment

because of the construction difficulties;

therefore, maintenance costs are higher.

Zirconium is even more expensive than titanium

and the construction requirements are also

higher, as both welding and forming have more

stringent purity requirements.

This aspect lays many doubts over the reliability

of a large welded construction made of this

material, which has rarely been attempted.

In any case the correct material selection by

itself does not guarantee a reliable performance

in the harsh environment of the urea synthesis.

Careful material screening and specific

construction procedures are keys requirements

to achieve the requested corrosion resistance.

Usually, detailed standards are developed by the

licensor specific for each type of material, to

guarantee the quality of the actual materials

used in construction and assure that its

characteristics do not deteriorate during the

manufacturing process.

Material selection is never separate from design,

because other parameters like mechanical

properties, workability and weldability as well

as economic considerations like prices,

availability and delivery have to be taken into

considerations. Even an accurate material

selection cannot by itself guarantee the

reliability of equipment, without correct design,

quality construction, and proper operation and

maintenance. In fact corrosion, even if under

control, is always present.

The special materials required for corrosion

resistance are not generally suitable for the

heavy-gauge construction needed to withstand

the high operating pressures for the dimensions

typical of modern plants. In general, such

equipment would be too expensive, and for

some materials the procedures involved in

fabrication would impair the corrosion-

resistance properties, such as stainless steel and,

especially, duplex steels.


For these reasons in nearly all applications the

corrosion-resistant materials are applied as a

lining to a pressure-resistant shell made of steel.

Different choices are available, related both to

the materials selected and to the design

philosophy.

The easiest way is to apply the corrosion

resistant layer directly to the pressure bearing

steel. That can be done by weld overlaying, or

cladding.

The main problem with this type of design is

that any damage to the protective lining cannot

be detected until also the pressure envelope is

corroded, with a high possibility of catastrophic

failure.

This solution has been used in the past, but it is

not generally considered nowadays for the

synthesis equipment. The common solution is to

apply the corrosion-resistant material as a loose

lining to the pressure casing. The loose lining

seals the corrosive fluid, while the external

casing has only a structural function. A leak

detection system is provided between the

internal layer and the pressure casing to inform

of any damage to the corrosion-resistant lining,

before the fluid can corrode the pressure

components.

This feature is essential for the reliability of the

equipment, since early detection of any failure

of the corrosion-resistant layer could prevent

damage to the pressure shell that could result in

the equipment collapsing or, at least, to damage

that is difficult to repair.

Automatic leak detection systems greatly

improves the safety and reliability of equipment,

permitting an early intervention, which usually

prevent extended damages difficult to repair.

Urea Reactor

The reactor is a simple vessel with adequate

volume to let the reaction progress, but the

pressure, the large dimensions and the huge

surface to be lined make it a critical

construction, where each detail must be

designed according to specific features to

guarantee the expected reliability.

The reactor has been layered with titanium,

zirconium, austenitic stainless steels (316L-UG

and 25Cr-22Ni-2Mo) and duplex stainless steel.

The choice depends on the type of process and

the specific experience of the licensor. 25Cr-

22Ni-2Mo has been the typical choice for

modern plant, giving excellent results in terms

of reliability and endurance.

Stripper

The stripper is possibly the most critical

equipment in the urea process.

Different process designs have different

requirements; that is why so many materials

have been used in the past.

The equipment itself is generally a falling-film

evaporator, and different details in design are

generally related to the different material

requirements.

Tube material is the most important choice.

Reliability is mainly connected to the specific

choice that each basic material selection

involves.

Notwithstanding the general similarity, each

basic material selection, from stainless steel to

duplex to titanium imply a specific detailed

design, where specific knowledge and

experience is the key for a trouble free life of

the equipment.

Carbamate Condenser The design of this equipment can vary

substantially according to the technology

selected.


Horizontal or vertical installation with process

fluid tube side or shell side are commonly used.

Since the reaction heat is usually utilized to

generate steam stress corrosion can be a

problem. In this case the use of a duplex

material for tubes may be advantageous.

Other Sections

The same principles for design and materials

selection should be used to guarantee the

reliability of the equipment in all other sections

of the plant, although fluids are less corrosive

and pressures lower.

In general, but not always, common stainless

steel and standard components can be used.

Only with experience and deep process

knowledge is it possible to know when this is

not true, making the difference between success

and failure. Sometimes more troubles arise in

this area than in the synthesis loop because less

diligence has been exercised in materials

selection.

Although common stainless steels can be used,

the appropriate limits on chemical composition

must be observed and quality checks carried out

in many components to avoid corrosion which,

when starts, can be fast.

A typical example is the use of welded stainless

steel tubes for exchangers. Whereas the base

material can be suitable for the operating

conditions, often the autogenous weld impairs

the corrosion resistance, leading to localized

corrosion (Figure 6). In general welded tubes

should not be used in most of the Urea plant

equipment.

Figure 6. Typical localized corrosion of a

welded tube

The same is valid for all component selection: in

general, standard components can be selected

but not all standard components are suitable.

A typical case is the hydrolyser in the waste

water treatment, where the stripping action of

steam is used to remove the NH3 and CO2 from

the treated urea plant waste water condensate so

as to maximize the hydrolysis of the urea

content.

That actually increases the corrosiveness of the

fluid.

Special test and chemical composition limits

should prescribed for the stainless steel

employed. Further tests should be provided on

the welds to assure the reliability of this

equipment.

3. Rotating Equipment Design

Several crucial choices must be made for the

selection of the proper equipment. Hereafter

some key points are discussed considering two

groups of machines: centrifugal compressors

and centrifugal pumps.


Centrifugal Compressors

The required process performances are always

the starting point for the compressor selection.

Other points that need to be considered during

the selection process are the following:

Operating range, turndown capacity and

anti-surge

A sufficient turndown capacity (Figure 7) is

required to permit an easy plant operation

in all cases (i.e. start-up, shut-down, SOR,

EOR, etc.) and to avoid as far as possible

the undesired opening of the anti-surge

valves. Anti-surge control system shall be

carefully designed and a sufficient number

of control loops for each compressor train

shall be considered.

Figure 7. Compressor performance curves

Material selection

Starting from the minimum process

requirements the compressor material

selection shall be properly checked

considering problems of corrosion (i.e. wet

CO2), but also allowable stresses. For this

last point special consideration shall be

given to impellers’ material. The tip speed

shall remain well inside the limits of the

applied material, of the references of the

selected vendor and of the applied

technology for the construction of the

impellers (i.e. welded, brazed or milled

from solid forgings).

Mechanical behavior

During the selection process the rotor

dynamic issues shall be carefully evaluated.

It is recommended to use always machines

referenced for the same application with

similar speeds and performances. At least a

preliminary evaluation of the critical speeds

and of the rotor stability shall be performed

during the evaluation process. Final reports

for lateral analysis, torsional analysis and

stability analysis shall be reviewed after

purchase order award to the selected

Vendor.

Quality control

The requirements for inspections and tests

shall be carefully defined during the design

phase.

Not only the final tests (i.e. mechanical

running test, performance test etc.) shall be

considered. Inspections and tests on the raw

materials (i.e. castings, forgings etc.) and

during the manufacturing process (i.e.

hydrostatic test, spin test, balancing test

etc.) shall be also taken into account.

Installation and commissioning

A proper installation and commissioning is

essential in order to achieve the best

possible mechanical behavior and

reliability. The required work hours for site

supervision shall be considered starting

from the design and tender phase.

Centrifugal Pumps At least the last two of the above points (i.e.

Quality Control and Installation and

Commissioning) are fully applicable also to the

centrifugal pumps.

Other key points for the pump selection are the

following:

Operating point

A pump is properly selected when the

operating point will fall into the Preferred

Operating Region, in other words when it is

close to Best Efficiency Point (BEP).


This is important not only for the efficiency

but also for the mechanical behavior (i.e.

lower vibration levels, longer life for

bearings and mechanical seals etc.).

NPSHa and NPSHr

Pumps shall be selected with a proper

margin between Net Positive Suction Head

available (NPSHa) and Net Positive Suction

Head required (NPSHr).

Pump selection can be affected by an

extremely low value of NPSHa. Some

service in ammonia and urea plants are

really sensitive to this matter and for them

pump selection shall be carefully evaluated

(i.e. Semi-lean Solution Pumps, HP

Carbamate Pumps).

Mechanical Seal Selection

The seal selection shall be done during the

design phase considering the properties of

the process fluid. The previous experience

for each specific service shall be also taken

into account.

Figure 8. Mechanical seal

For some services the installation of double

pressurized seals with flushing from an

external source is required (i.e. HP

Ammonia Pump for urea plants). This type

of seal arrangement is the one that

guarantee the maximum level of reliability,

safety and environmental protection.

For those services having fluids that can

crystallize but, where single mechanical

seals (Figure 8) can be applied (i.e. melt

urea, urea solution, etc.), a proper selection

of flushing plan is necessary. For these

service Urea CASALE requirement is to

apply an internal recirculation (API Plan

01) in conjunction with steam (or hot water)

quench (API Plan 62) to the atmospheric

side of the seal (Figure 9).

For melt urea, a steam jacket is also

required.

Figure 9. Flushing plans (Source: John Crane)

4. Instrumentation Design

The reliability of ammonia and urea plants is a

critical goal to meet during the development of

the engineering. A good and operator friendly

control philosophy is the first issue to address.

The operator interface is a very important issue

to take in consideration. It shall be easy to read

and simple to operate in order to minimize the

intervention in case of problems.

The redundancy of the control and safety

systems such as DCS or ESD in various aspects

is the way to balance the availability and the

safety of the plants.


The minimum requirements of redundancy on

DCS are concentrated in the common parts plus

those parts related to critical controls. ESD is

typically redundant but final safety strategy

needs are defined with dedicated SIL study.

For field instrumentation attention shall be paid

to critical High Pressure Control Valves for

Urea service where type of material, particular

design of the trim and body, jackets, heating and

experiences of specialized vendors are the basis

to guarantee a good result. Those valves are

the result of years of operation and modification

to meet good a reliability required on those

types of plants.

Other critical devices are the Safety Relief

Valves on urea service where heating and

flushing is a must to obtain a high reliability and

safety of the plant.

A steam injection is necessary for preventing

crystallization in the seat of the valve and

discharge piping (Figure 10).

Figure 10. Relief valve for urea application

In general, for transmitters, the attention shall be

given to instruments with diaphragms and

capillary, as they are not simple to bypass in

case of malfunctioning. Special care is required

during construction and tests are necessary to

verify the real conformity with the

requirements.

Critical instruments are in the area of urea

reactor, stripper, compressor and pumps

discharge.

Engineering experience on these types of plants

is a must to develop a good detail and a choice

of the dedicated valves and instruments.

5. Electrical Design

Ammonia and Urea pants operation and

reliability depend on safe and efficient operation

on their electrical supply equipment.

One of most important issues of the electrical

system in ammonia and urea plants is related to

the necessity of installing very large motors

which may require direct-on-line starting. In

particular, when compressors are not driven by

steam turbine, for example the CO2 compressor

in Urea plant and Natural Gas and Syngas

compressors in Ammonia plant, they are

normally driven by very large induction or

synchronous electrical motors.

In that case, the network study is the best

solution to analyse all the aspects related to the

behaviour of the electrical distribution system.

The results of load flow study, short circuit

calculation, power system dynamic calculation,

harmonic analysis, protection coordination and

discrimination studies shall be carefully

examined during engineering stage and

appropriate solutions shall be adopted to

anticipate any motor starting complications or

undesired disturbing of external network which

can be revealed during commissioning phase or

when the plant is in operation.

The arrangement of the power distribution and

control system shall be defined matching the

process and operating requirements with the

most modern electrical equipment and control

philosophy.

For the power distribution, redundant feeding

line is the typical supply concept adopted for the

Urea and Ammonia plants.

In case of power failure of both incoming

feeders, an emergency feeding line (coming

Steam

injection


from external source or from a dedicated

Emergency Diesel Generator) shall

automatically feed the critical loads necessary to

shut-down the plant and keep it in safe

conditions. Automatic restarting of essential

motors is usually applied to make operators task

easier.

On the other hand, high performance automatic

transfer systems with suitable interlock/inter-trip

and Power Distribution Control System

(PDCS), based upon one programmable logic

controller, represent the basis of the control

system.

6. Piping Design

Regarding plot plan development, engineers

have today all the necessary instructions from

dedicated and complete procedures and

specifications. Moreover using modern

technologies like 3D software and internet, all

the aspects of safety, ergonomic,

constructability, cost saving can be analyzed

and solved by dedicated teams of engineers.

Then we can say that almost everything is

already written in the literature, we have just to

apply it.

We highlight just one of the aspects that most of

the times is not sufficiently considered. We are

speaking of developing and handling over a

plant where the maintenance can be done using

the staff normally dedicated to this activity in

the modern plants.

Since technologies have not been substantially

modified in the years and Companies have

developed similar plants, layouts for fertilizer

plants have been consolidated, compacted and

extremely rationalized during the years, using or

cutting any available room in order to reduce

construction costs. Most of the times, for Clients

this means tight access to equipment, valves or

instruments and additional manpower and

devices are always required for the purpose,

increasing costs and time. Also equipment, that

is known to be replaced after a limited number

of years, is sometimes placed where their

removal is critical.

Then in the ideal plant any heavy removable

part, such as equipment or part of equipment,

instruments, valves, devices, should be studied

and places in such a way to be accessible and

dismantled using a crane. This method allows a

small team to easily and safely maintain their

plants.

7. Conclusions

This paper has described all the aspects relevant

to reliability that are always taken in

consideration by CASALE during the design

stages of ammonia and urea plants in addition to

process performance.

The design of a new plant requires deep

knowledge on all engineering disciplines, about

possible problems as well as a specific

experience in the relevant process. CASALE as

engineering company has in-house specialist

able to manage in an efficient and professional

way all the issues related to plant reliability.

Moreover, CASALE, as a supplier of a wide

range of proprietary equipment, has developed

several reliable solutions that best suits the

needs required both for plant retrofitting and for

new plants.


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